Silicon-on-Insulator (SOI) technology is of growing importance in the field of integrated circuits. SOI technology involves forming transistors in a relatively thin layer of semiconductor material overlying a layer of insulating material. More particularly, SOI technology is characterized by the formation of a thin silicon layer (device region) for formation of the active devices over an insulating layer, such as an oxide, which is in turn formed over a substrate. Transistor sources and drains are fonned, for example, by implantations into the silicon layer while transistor gates are formed by forming a patterned oxide and conductor layer structure.
Such structures provide a significant gain in performance by having lower parasitic capacitance (due to the insulator layer) and increased drain current due to floating body charging effects (since no connection is made to the channel region and charging of the floating body provides access towards a majority of carriers which dynamically lower the threshold voltage, resulting in increased drain current). Devices, such as metal oxide silicon field effect transistors (MOSFET), have a number of advantages when formed on SOI wafers versus bulk silicon MOS transistors. These advantages include: reduced source/drain capacitance and hence improved speed performance at higher-operating frequencies; reduced N.sup.+ to P.sup.+ spacing and hence higher packing density due to ease of isolation; absence of latchup; lower voltage applications; and higher "soft error" upset immunity (i.e., the immunity to the effects of alpha particle strikes).
Although there are significant advantages associated with SOI technology, there are significant disadvantages as well. For example, poor heat removal from devices on an SOI substrate is a significant disadvantage. Electrical devices generate heat, and the inability to remove or dissipate the heat results in poor and/or inconsistent performance of the electrical devices, or even in some instances device and/or substrate degradation.
There is poor heat removal for devices on SOI substrates primarily because of the oxide insulation layer. This is because the oxide insulation layer has a markedly lower thermal conductivity than the thermal conductivity of conventional bulk silicon (typically used as semiconductor substrates), which typically surrounds semiconductor devices. As a result, the buried oxide layer undesirably thermally insulates the device region in SOI substrates.
In view of the aforementioned disadvantages, there is a need for SOI devices of improved quality, particularly SOI devices having improved heat removal characteristics, and more efficient methods of making such SOI devices.